Elastic cartilage

Elastic cartilage is found where support with flexibility is required, as in the ear, auditory tube, and epiglottis. Basically, elastic cartilage is identical to hyaline cartilage except that, in addition to collagen type II fibrils, it contains an abundant network of fine elastic fibers, which may be demonstrated by standard elastin stains.

The chondrocytes of elastic and hyaline cartilage tissues are similar, and elastic cartilage is frequently found to be gradually continuous with hyaline cartilage. Like hyaline cartilage, elastic cartilage possesses a perichondrium. The presence of elastic fibers in the extracellular matrix makes elastic cartilage somewhat less susceptible to degenerative processes than hyaline cartilage.

Fig.23. Elastic Cartilage

Fibrocartilage has characteristics intermediate between those of dense connective tissue and hyaline cartilage. It is found in intervertebral disks, in attachments of certain ligaments to the cartilaginous surface of bones, and in the symphysis pubis. Fibrocartilage is always associated with dense connective tissue, and the border areas between 2 tissues are not clear-cut but show a gradual transition. The chondrocytes are scattered in rows between the collagenous fibers (collagen type I)

Fig.24. Fibrocartilage

BONE

Bone is one of the hardest tissues of the human body.

As the main constituent of the adult skeleton, it supports fleshy structures, protects such vital organs as those in the cranial and thoracic cavities, and harbors the bone marrow, where blood cells are formed.

Bone also serves as a reservoir of calcium, phosphate, and other ions that can be released or stored in a controlled fashion to maintain constant concentrations of these important ions in body fluids.

In addition to these functions, bones form a system of levers that multiply the forces generated during skeletal muscle contraction, transforming them into bodily movements.

Bone is a specialized connective tissue composed of intercellular calcified material, the bone matrix, and 3 different cell types:

• osteocytes, which are found in cavities (lacunae) within the matrix;

• osteoblasts, which synthesize the organic components of the matrix;

• osteoclasts, which are multinucleated giant cells involved in the resorption and remodeling of bone tissue.

Bone Matrix. Bone has considerably more collagen content than does cartilage (25-30% of the total organic material). The collagen fibers are much more orderly in their arrangement as well. The collagen fibers give bones the ability to resist snapping and breaking.

• The mineral component, which gives bone its hardness, is chiefly a form of calcium phosphate, called hydroxyapatite. Significant quantities of amorphous (noncrystalline) calcium phosphate are also present.

• The organic matter is type I collagen and amorphous ground substance, which contains glycosaminoglycans associated with proteins.

• Several specific glycoproteins have been isolated from bone. Bone sialoprotein and osteocalcin contain several acid residues; this causes them to bind calcium avidly and may be responsible for promoting calcification of bone matrix.

The association of hydroxyapatite with collagen fibers is responsible for the hardness and resistance that are characteristic of bone. After a bone is decalcified, its shape is preserved, but it becomes as flexible as a tendon. Removal of the organic part of the matrix also leaves the bone with its original shape; however, it becomes fragile, breaking and crumbling easily when handled.

CELLS

Osteoblasts are responsible for the synthesis of the organic components of bone matrix (type I collagen, proteoglycans, and glycoproteins). Deposition of inorganic components of bone is also dependent on the presence of viable osteoblasts. They are exclusively located at the surfaces of bone tissue, side by side, in a way that resembles simple epithelium. When they are actively engaged in matrix synthesis, osteoblasts have a cuboidal to columnar shape, basophilic cytoplasm, and high alkaline phosphatase activity.

Once surrounded by newly synthesized matrix, the osteoblast is referred to as an osteocyte. Lacunae and canaliculi appear, because the matrix is formed around a cell and its cytoplasmic extensions.

During matrix synthesis, osteoblasts have the ultrastructure of cells actively synthesizing proteins for export. Secretion of matrix components occurs at the cell surface, which is in contact with older bone matrix, producing a layer of new (but not yet calcified) matrix – osteoid - between the osteoblast layer and the previously formed bone (Fig.25).

Fig.25

This process, bone apposition, is completed by subsequent deposition of calcium salts into the newly formed matrix

Дата добавления: 2015-10-31